As noted above, flake pigments are substances which affect the reflection, transmission and absorption of light when incorporated in a binder and applied to a surface. These flakes are in the shape of thin plates having a nominal thickness from approximately 50 angstroms to 300 microns and generally possess diameters from several to several hundred microns. As taught in U.S. Pat. No. 3,123,490 to Bolomey et al., the disclosure of which is hereby incorporated by reference, these plates can be composed of either a single material or two or more materials arranged in layers.
When flake pigments are incorporated in a binder and applied to a surface, the flakes tend to lie parallel to one another and to the surface. If the flake index of refraction differs from that of the binder, light is reflected at the binder-flake interface which can provide a nacreous pearlescent luster as again taught by Bolomey et al. When the flakes are composed of multilayer films, with the index of refraction changing from layer to layer in accordance with well known techniques for designing thin film interference structures, the spectral character of their reflectivity can be controlled. Furthermore, the reflection changes spectrally as the viewing angle is changed. Thus, the surface might appear red when viewed at normal incidence and blue when viewed at, for example, an angle 45.degree. from normal. For example, Bolomey et al. teaches using such pigment flakes to simulate pearl buttons by incorporating the flakes in a polymethylmethacrylate binder and by applying the resulting lacquer to glass beads to simulate natural pearls.
Pigment flakes can also be employed in the production of anti-counterfeiting inks or coatings. In such an environment, multi-layered pigment flakes are incorporated in a binder and the mixture used as an ink. The color of the ink is determined by the thicknesses, number, and indices of refraction of the layers in the flake structure. Obviously, color change as the viewing angle changes and this characteristic can be used to authenticate articles ranging from currency to blue jeans.
Previously, pigment flakes have been produced employing vacuum evaporation techniques. Such techniques can be employed to both deposit a film and to aid in removal of the film from a substrate in flake form. For example, Bolomey et al. teaches the use of an endless belt of 25 mil thick polyester film mounted on two parallel horizontal rollers in a vacuum chamber. This belt moves serially over four boats loaded with Na.sub.2 B.sub.4 O.sub.7, ZnS, MgF.sub.2, and ZnS, respectively. As a specific section of the belt moves over a particular boat containing one of the above-enumerated compounds, it is coated with a layer of the material in that boat resulting in a ZnS/MgF.sub.2 /ZnS interference coating on top of a Na.sub.2 B.sub.4 O.sub.7 layer which is to act as a release layer. The thickness of the flake layers can be adjusted to yield the desired reflectance and transmittance spectra.
After the coating of Bolomey et al. is applied and the coating and its substrate brought to atmospheric pressure, the belt is removed from the vacuum coater and washed with water which dissolves the release layer and releases the film from the polyester belt as flakes. These flakes are washed to remove the release layer and are then filtered and dried.
Yet another technique for making pigment flakes is taught by Ash et al. in U.S. Pat. No. 4,434,010. In this instance, an interference film is coated on a polyethylene substrate in a large vacuum chamber. After removal from the chamber, the film is removed in flake form by dissolving the substrate in acetone or some other suitable solvent. Again, the flakes must be filtered and dried.
It is quite evident from a consideration of the techniques taught by Bolomey et al. and Ash et al. that the production of pigment flakes in large quantities is a formidable task requiring large, expensive vacuum equipment and further requiring the removal of the pigmented film from its substrate by a wet process carried out outside the vacuum chamber. As such, it is evident that the vacuum chamber must first be subjected to low pressure, the film deposited on a substrate followed by venting the chamber to ambient conditions, whereupon the substrate must be exposed to either acetone or similar solvent for the release layer necessitating a further filtering and drying process to remove the flakes from the dissolving medium.
The speed of prior art processes is obviously limited by a number of factors. These include the mass of flake producing material which can be deposited in a given time in a roll coater which is in turn governed by the web speed and thicknesses of the layers comprising the flakes. Using Bolomey et al. as an example, employing a substrate width of 1 foot and deposition rates as provided by Bolomey et al., mass deposition of 0.1 to 0.2 gm/min can be expected. Employing a webbing width of 5 feet, which is typical of a very large roll coater, the rate only becomes 0.5 to 1 gm/min. An examination of the prior art, generally, indicates that such rates are typical for the vacuum roll coaters depositing dielectrics in a fashion which allows good thickness control.
It is thus an object of the present invention to provide pigment flake material while avoiding the disadvantages inherent in practicing those prior art methods described above.
It is yet another object of the present invention to produce pigment film material at a relatively high rate using inexpensive equipment.
It is still a further object of the present invention to provide an efficient method for removing thin film material in the form of flakes from a substrate on which it was deposited.